Twin refiner with double floating discs

ABSTRACT

A high speed ground wood disc refiner having a pair of oppositely facing fixed refiner plates and twin rotating discs each carrying refining plates juxtaposed the fixed plates. The rotating discs are axially adjusted on the drive shaft by a parallel piston-cylinder hydraulic system which insures that an equal and opposite thrust force is produced by each rotating head. Since the thrust forces are absorbed by the drive shaft, this arrangement permits the use of very light bearings on the drive shaft, resulting in economies of manufacture and maintenance.

United States Patent [1 1 Fisher et al. July 8, 1975 [54] TWIN REFINER WITH DOUBLE FLOATING 3,276,70l l/l966 Fisher 241/2592 X DISCS 3,717,308 2/1973 Reinhall 241/244 3,810,584 /1974 Kahmann 24l/37 [75] Inventors: Chester Donald Fisher; B atri 3,841,573 /1974 Kahmann 241/245 Pearl Fuller, both of Muncy, Pa. [73] Assignee: Sprout, Waldron & Company, Inc., 'f Examliler eranvme Custer Muncy, pa Asststant ExammerHoward N. Goldberg Attorney, Agent, or Firm-Howson and Howson [22] Filed: July 17, 1974 [21] Appl. No.: 489,416 [57] ABSTRACT A high speed ground wood disc refiner having a pair 52 us. c1. 241/37; 241/146; 241/247; PP Y facing fixed I and 24] ,1 241/285 A ing discs each carrying refining plates juxtaposed the [51] Int. Cl. B02C 7/14 fixed Plates The rotating discs are axially adjusted on 5 Field of Search 241 37 4 188 A, 244 the drive shaft by a parallel piston-cylinder hydraulic 241/245-247 2591 12592 23 5 A system which insures that an equal and opposite thrust force is produced by each rotating head. Since the [56] References Cited thrust forces are absorbed by the drive shaft, this arrangement permits the use of very light bearings on UNITED STATES PATENTS the drive shaft, resulting in economies of manufacture 2,437,147 3/1948 Zwoyer 241/146 and maintenance 2,971,704 2/l96l .lohansson... 24l/259.l X 3,038,673 6/1962 Fisher 24l/245 13 Claims, 6 Drawing Figures If] /4 la 1" a, i r

1 r" l \Q 52 I25 84 I! an 6 l l1 l 1' ll & 5 l 42 m .vz /f In :4 7a a [fill I I PATENTEDJUL 8 ms SHEET 10F 3 FIGI.

1 TWIN REFINER WITH DOUBLE FLOATING DISCS The present invention relates generally to disc type ground wood refiners and relates more particularly to a high speed twin refiner having double rotating floating discs.

Disc type refiners are well known, particularly in the paper making field, and essentially comprise juxtaposed annular refining surfaces which are arranged for relative rotation so that material to be refined introduced at the inner edge of the surfaces is transported by centrifugal force between the surface and peripherally discharged in a refined condition. The refiners commonly comprise either a single pair of refining surfaces or, in the twin refiners, a double pair of refining surfaces. One type of single refiner is characterized by a rotationally and axially fixed refiner head which cooperates with an axially adjustable rotatable head. In another type of single refiner, the heads are counterrotating with one being axially fixed and the other being axially adjustable. A further type is characterized by a rotating head which is axially fixed and a nonrotating head having means for axial adjustment to change the refining plate clearance.

The twin refiners typically include a central rotating head having attrition plates on each side thereof cooperating with oppositely disposed rotationally fixed plates. One or both of the heads supporting the fixed plates may be axially adjustable to vary the plate spacing, and the shaft carrying the rotating head may be either axially adjustable or freely floating. An example of the freely floating type twin disc refiner having one of the fixed heads axially adjustable is shown in U.S. Pat. No. 3,276,701, assigned with the present application to a common assignee.

As the refiners increase in size, the thrust forces produced by the refining action become quite large. Refiners are currently being built with plate diameters of over four feet and horsepower requirements exceeding 1,000 horsepower. The conventional twin disc refiners have to a large extent eliminated the problem of axial thrust forces on the rotating shaft, and this is particularly true of the floating disc refiners of the type shown in U.S. Pat. No. 3,276,701. A difficulty, however, with the floating type twin refiner is the danger of uneven stock feed resulting in a drift of the floating head and the possibility of plate clashing. Furthermore, such an imbalance could produce at least momentary high thrust forces on the shaft and suitable bearings must be provided for such a contingency.

ln the present invention, a twin refiner is provided having a pair of non-rotating axially fixed stationary heads and a pair of axially adjustable rotating heads disposed therebetween. Refining plates mounted on the stationary and rotating heads cooperate in the conventional manner to carry out an attrition action on materials passing therebetween. The rotating heads are each mounted on sleeves axially slidable on the drive shaft and are driven in rotation by a central driving disc on the drive shaft having driving lugs extending from each side thereof engaging similar lugs on the rotating heads. The sleeves supporting the rotating heads are axially positioned by cylinder-piston means connected to a parallel hydraulic circuit whereby thrust forces developed by the rotating heads are equalized. Means associated with the cylinder-piston means restrict the axial movement of the rotating heads and prevent the possibility bf plate clashing, thereby allowing operation of the refiner under no-load conditions with a predetermined clearance between the refining plates.

It is accordingly a first object of the present invention to provide a high speed twin disc refiner for refining ground wood and the like of a novel improved design.

A further object of the invention is to provide a twin disc refiner as described having double rotating floating discs which minimize thrust forces transmitted by the refiner shaft thereby permitting the use of very light shaft bearings.

A further object of the invention is to provide a twin disc refiner as described having means for limiting the floating movement of the rotating discs to prevent plate clashing and permit running of the refiner even under no-load conditions.

Still another object of the invention is to provide a twin disc refiner as described which compared to conventional refiners is substantially more economical to build and to maintain.

Further objects and advantages of the invention will be more readily apparent from the following detailed description of an embodiment thereof when taken together with the accompanying drawings wherein:

FIG. 1 is a side elevational view of a refiner constructed in accordance with the present invention;

FIG. 2 is an end elevational view looking into the righthand end of the refiner shown in FIG. 1;

FIG. 3 is an enlarged sectional view taken along line 3-3 of FIG. 2 and showing the interior details of the refiner;

FIG. 4 is an enlarged partial sectional view with a portion broken away and in section of the righthand cylinderpiston arrangement shown in FIG. 3 for positioning the righthand rotating disc;

FIG. 5 is a sectional view taken along line 5-5 of FIG. 3; and

FIG. 6 is a partial plan view taken along line 6-6 of HG. 3 showing the manner in which the lugs of the driving disc cooperate with the lugs of the rotating discs.

Referring to the drawings and particularly FIGS. 1-3 thereof, a refiner generally designated 10 in accordance with the present invention comprises a base 12 supporting spaced separable casing sections 14 and 16. A central casing section 18 is removably secured between the sections 14 and 16 to provide access to the refiner plates. The casing section 18 may comprise a pair of arcuate portions which are clamped between the casing sections 14 and 16 by the bolts 20 joining the sections.

Bearing support portions 22 and 24 respectively extend outwardly from the casing sections 14 and 16 to rotatably support the refiner drive shaft 26 which extends through the casing sections. With reference to FIG. 3, the lefthand end of the drive shaft 26 has a stepped configuration journalled in bearing assembly 27 secured within the bearing support portion 22. The drive shaft is axially located by the bearing assembly 27 which abuts against a shoulder 28 of the drive shaft and is maintained thereagainst by the lock ring 30 threadedly mounted on the drive shaft. The bearing assembly 27, which is seated within a shouldered bore 32 of the cylindrical support member 34, is secured in place therein by the annular locking member 36 and the lock ring 38 fastened by screws 40. Seal means 42 and 44 at opposite ends of the bearing assembly 27 prevent foreign matter from reaching the bearings. The bearings as illustrated are of the angular contact type which will absorb light thrust forces although as described below, the refiner under normal operation should not generate any substantial thrust forces which need be absorbed by the refiner shaft bearings.

The drive shaft at the opposite end of the refiner is supported by the bearing support portion 24 within which the cylindrical support member 46 is secured. A radial bearing assembly 48 is mounted in the shouldered bore 50 of the member 46 and secured in position by the annular ring 52. Seal assemblies 54 and 56 are provided on either side of the bearing assembly 48.

The casing sections l4, l6 and 18 describe surfaces of revolution around the axis of the shaft 26 which is considered the refiner axis. Circular openings 58 and 60 respectively in the ends of casing sections 14 and i6 permit the passage of the shaft 26 therethrough. As shown particularly in FIG. 2. the base 12 is provided with an arcuate contoured surface 62 on which the easing sections 14 and 16 are slidabiy supported. Although the casing sections are secured together by the bolts during operation of the refiner, for plate inspection or replacement. the casing sections 14 and 16 may be axially moved to the positions 14' and 16' illustrated in dot and dash lines in FIG. 3. To move the casing sections to the opened position, piston-cylinder assemblies 64 and 66 secured by brackets 64a and 66a to the base are respectively connected with the casing sections 14 and 16 to provide the axial force required to slide the casing sections on the arcuate surface 62 of the base.

The casing sections l4 and 16, which are frustoconical in shape as shown in FIG. 3, respectively include non-rotating axially fixed heads 68 and 70 diaposed perpendicularly to the drive shaft axis. The heads are of an annular shape, being secured to the casing sections at their outer radial edge and being secured at their inner edge to the cylindrical stock conduit members 72 and 74 disposed concentrically in spaced relation about the drive shaft 26. Refiner plate assemblies 76 and 78 are respectively mounted on the opposed ra disi faces of the heads 68 and 70. The refining plate assemblies esch comprise a conventional array of refln ing plate elements such as those shown in US. Pat. No. 3.473.745, assigned with the present application to a common sssignee.

A driving disc 82 is secured to the drive shaft 26 in a radially aligned central position equidistant the stationsry heads 68 and 70. The driving disc 82 includes a plurality of uniformly spaced lugs 84 extending axially from each side of the periphery thereof as shown in FIGS. 3 and 6.

Rotating heads (also known as discs) 86 and 88 are disposed on opposite sides of the driving disc 82 respsctivsly juxtaposed the non-rotating heads 68 and 70. The rotating heads 86 and 88 are generally annular in shape and are respectively mounted on the sleeves 90 and 92 which overlie the drive shaft 16 in close fitting but axially slldsbie relation thereto. The rotating heads 86 and 88 and hence the attached sleeves 90 and 92 are driven in rotation by the driving disc 82 by means of lugs 94 and 96 extending respectively from the heads 86 and 88 which engage the lugs 84 of the driving disc. This lug arrangement effects a rotation of the rotating heads with the drive shaft but permits the axial movement thereof since the lugs of the rotating heads may slide axially along the lugs of the driving disc.

Refining plate assemblies 98 and 100 are respectively secured to the rotating heads 86 and 88 in juxtaposed relation to the refining plate assemblies 76 and 78 of the stationary heads 68 and 70. The refining plate assemblies 98 and 100 in the normal operating position of the rotating heads are spaced at small fraction of an inch from the stationary refining plate assemblies 76 and 78 to effect an attrition effect on the stock passing therebetween. The spacing of the plates is controlled by adjusting the position of the sleeves 90 and 92 as described below.

Stock feed conveyor assemblies 102 and 104 are respectively mounted on the casing sections 14 and 16. Since the feed conveyors 102 and 104 are essentially identical aside from the direction of rotation of the conveyor screws, only the conveyor assembly 104 will be described in detail. With reference to FlGS. 2 and 3, the conveyor assembly 104 includes a horizontal cylindrical conveyor casing 106 which extends from a vertical flanged input conduit 108 exteriorly of the easing section 16 to a downwardly directed discharge chute l 10 within the casing section vertically above the refiner axis. The discharge chute U0 connects with opening 112 in the cylindrical casing member 74. A conveyor screw 114 mounted within the conveyor casing 106 is driven in rotation by a motor 116 connected thereto by chain drive assembly 118 as shown in FIG. 2. Stock directed into the input conduit 108 will accordingly be advanced by the conveyor screw 114 to the discharge chute I10 and pass through opening 112 in the member 74 into the annular sectioned chamber [20 formed between the sleeve 92 and the member 74. A similar chamber 122 is formed at the opposite end of the refiner and stock is fed thereto by the conveyor assembly 102 in the identical manner to that described with respect to conveyor assembly 104. The rotation of the conveyor screws is such as to direct the stock toward the rotating heads 86 and 88 and thus as viewed in FIG. 3 the conveyor screw 114 is rotated in a clockwise direction while the screw of the conveyor assembly 102 will rotate in a counterclockwise direction.

The feed of the stock from the chambers 120 and 122 respectively into the refiner throat areas 124 and 126 adjacent the refining plate assemblies is effected by conveying means comprising a plurality of triangular lugs 128 rotating with the drive shaft and extending into the chambers 120 and 112. The triangular lugs 12B are disposed so that angularly oriented faces thereof will direct the stock toward the throat regions 124 and 126 regardless of the direction of rotation of the drive shaft. The reversibility of the drive shaft is desirable to permit a uniform wearing of the attrition elements of the refiner plates. The lugs 128 are preferably arranged in evenly spaced relation in axially aligned rows with the lugs of each row being axially staggered with respect to the lugs of the adjacent rows. The lugs 12B are secured to sleeve extensions 90a and 92a which overlie and extend axially outwardly beyond the sleeves 90 and 92.

The lugs 128 permit the passage of steam generated in the refining process bacit through the incoming stock to steam discharge conduits 130 and 132 which pass respectively vertically thorugh the casing sections 14 and 16 at the outermost ends thereof. The reversible lug conveyor construction illustrated is in accordance with the disclosure of copendlng application Ser. No. 296,564, now U.S. Pat. No. 3,853,276, assigned with the present application to a common assignee. Although this is the preferable stock conveyor construction in view of its reversibility, a ribbon screw refiner feeder could also be utilized such as that shown in U.S. Pat. No. 3.441 ,227 also assigned with the present application to a common assignee. Either of these conveyors permits the desirable reverse flow of steam which is generated in the refining of high consistency stock.

The refined stock passes from between the sets of refining plates into an annular chamber defined by the casing intermediate section 18, the stationary heads 68 and 70, and the rotating heads 86 and 88. Some refined stock may find its way between the driving disc 82 and the rotating discs 86 and 88 although the bulk of the refined stock will be thrown by centrifugal force into the peripheral regions of the chamber 134. A discharge conduit 136 communicating with the annular chamber 134 passes through the base 12 for discharge of the refined stock.

The hydraulic system for axially positioning the rotating heads 86 and 88 comprises a coaxial bore 138 in the drive shaft 26 extending from the lefthand end thereof as viewed in FIG. 3 to a point approximately even with the outer end of the casing section 16. The bore 138 includes a pair of cylindrical plugs 140 and 142 disposed proximate the outer ends of the sleeves 90 and 92 respectively. Since the construction and function of each of the plugs 140 and 142 as well as the passages in the drive shaft adjacent the plugs is essentially the same, only the plug 142 and the cooperative hydraulic passages as shown in the enlarged view of FIG. 4 will be described.

The bore 138 is threaded at its open end 144 for connection to a source of pressurized fluid (not shown}. The hydraulic fluid in the bore 138 passes through passages 146 in the plugs 140 and 142, thus filling the bore 138 throughout its length. Radial passages 148 and 150 connect the bore 138 with chambers 152 and 154 respectively formed adjacent the drive shaft 26 by the pistons 156 and 158 which are threadedly secured to the drive shaft, and the flanged ends 160 and 162 of the sleeve extensions 90a and 92a. The introduction of pressurized hydraulic fluid into the chambers 152 and 154 acts to move the rotating heads axially toward the driving disc 82 and hence away from the stationary heads to increase the plate clearance.

Chambers 164 and 166 are formed on the outer sides of the pistons 156 and 158 by the flanged end cover members 168 and 170 which are secured to the flanged ends 160 and 162 of the sleeve extensions 90a and 92a respectively. The end cover members 168 and 170 are in sealing engagement with the drive shaft 26 and accordingly, the introduction of pressurized hydraulic fluid into the chambers 164 and 166 will result in the outward movement of the floating heads to decrease the plate clearance. Pressurized hydraulic fluid is introduced to the chambers 164 and 166 through diagonal passages 172 and 174 respectively which intercept the bore 138 at the plugs 140 and 142, communicating with an annulus 176 therein. As shown in FIG. 4, each annulus 176 communicates in turn with radial passage 178 leading to axial passage 180 of the plug. A conduit 182 passing axially through the bore 138 is threadedly connected to the plugs to communicate with the axial bores therein. The conduit 182 terminates in a threaded connector 184 for connection with a pressurized hydraulic source (not shown).

The described hydraulic arrangement for controlling the axial position of the rotating plates is essentially a piston-cylinder arrangement wherein the pistons are fixed and the cylinders move with respect to the fixed pistons. The travel of the rotating heads 86 and 88 is respectively limited in the closing direction by stop rings 186 and 188 which are threadedly adjustable on the drive shaft and locked in the desired position by lock screws. Flanged stop nuts 190 and 192 threadedly attached to the end covers 168 and 170 limit the opening movement of the rotatable heads 86 and 88 respectively.

For operation, the refiner drive shaft 26 is connected by means of extending shaft end 26a to a power source, normally an electric motor of substantial size, for example 1,000 horsepower. The stock input conduits are connected with a source of stock to be refined which typically flows under gravity force from an elevated location into the feed conveyors 102 and 104. The drive shaft bore 138 and the conduit 182 are connected with appropriate sources of pressurized hydraulic fluid through control and valve means which will connect one of the channels to drain while the other is connected to the pressurized fluid source.

The feed conveyors 102 and 104 are driven in rotation at a predetermined speed to introduce a uniform flow rate of stock into the chambers and 122. The stock is directed into the throat regions 124 and 126 by the triangular lugs 128 and is centrifugally advanced between the refiner plates in a well known manner. The refined stock is discharged from the chamber 134 through the discharge conduit 136. Steam which may develop during the refining process passes from the throat regions through the chambers 120 and 122 and is discharged through the discharge conduits and 132. Although the steam is travelling against the infeed of stock, the stock will be centrifugally thrown against the outer walls of the chambers 120 and 122, permitting the steam to pass along the inner chamber walls without interfering with the stock flow.

The control of the plate spacing is accomplished by means of the cylinder-piston assemblies acting in parallel to axially adjust the position of the rotating heads 86 and 88. The application of pressurized hydraulic fluid to the chambers 152 and 154 while opening the chambers 164 and 166 to drain will result in an increase in the plate spacing. Conversely, an application of pressurized hydraulic fluid to the chambers 164 and 166 and a draining of the chambers 152 and 154 will decrease the plate spacing. Since the recited chambers are connected in parallel with the sources of hydraulic pressure, the rotating heads 86 and 88 are freely floating on the drive shaft 26 and will become positioned so that the forces developed between the two sets of refining plates are equal inasmuch as the opposing forces provided by each of the piston-cylinder assemblies is equal.

The use of a parallel hydraulic system to govern the plate spacing results in an equal and opposite thrust being exerted on the piston rings 156 and 158 during the operation of the refiner. Since this thrust is absorbed by the refiner shaft, the re is no need for any substantial shaft thrust bearings other than those necessary to support incidental loads. Considering the enormous horsepower input commonly utilized in twin refiners, the elimination of sizable thrust bearings and their maintenance comprises a substantial saving in contrast to conventional refiner construction.

An advantageous feature of the cylinder-piston control of the rotating heads is the stop arrangement which prevents any possibility of plate clashing, even if the refiner is run without stock flow. The stop rings 186 and 188 are threadedly adjusted in position and locked on the drive shaft so that the end covers 168 and 170 respectively will engage the stop rings before the refining plates clash. Similarly, the flanged stop nuts 190 and 192 limit the degree to which the refining plates can be separated since the flanges of the stop nuts will engage the stop rings at a predetermined point.

Although in the preferred embodiment of the invention illustrated the rotary heads 86 and 88 are driven in rotation by the driving disc 82 secured to drive shaft 26, it will be obvious that other driving means could be employed for this purpose. For example, keys or splines could be provided on the drive shaft for cooperation with appropriate slots or grooves on the rotating heads. The present arrangement is preferred, however, since the driving force between the driving disc lugs and rotating heads is minimized by establishing their contact at a maximum radius from the drive shaft. The minimal contact force between the lugs reduces the axial force required to move the rotating heads, thus improving the floating" action of the rotating heads.

Manifestly, changes in details of construction can be effected by those skilled in the art without departing from the spirit and the scope of the invention.

We claim:

1. A disc type refiner comprising a base, a casing supported by said base, a drive shaft extending through said casing, means on said casing for rotatably supporting said drive shaft, a pair of stationary heads mounted within said casing in spaced opposed relation, a refining plate assembly radially disposed on each said stationary head. a pair of rotating heads mounted on said drive shaft, a refining plate assembly on each said rotating head in parallel juxtaposed relation to the refining plate assembly of the adjacent stationary head, means connecting said drive shaft and said rotating heads to provide rotation of said heads with said shaft while permitting axial movement thereof along said shaft, conveying means for directing stock to be refined into said casing between said juxtaposed refining plate assemblies, and means for adjusting the axial position of said rotating heads on said drive shaft, said latter means comprising cylinder-piston means on said shaft connected with each said rotating head, and a parallel hydraulic circuit operatively connecting said piston-cylinder means to equalize the positioning force on each said rotating head thereby permitting said rotating heads to float axially to balance the hydraulic cylinder-piston force against the axial force developed between the refining plate assemblies.

2. A disc type refiner comprising a base, a casing supported by said base, a drive shaft extending through said casing, means on said casing for rotatably supporting said drive shaft, a pair of stationary heads mounted within said casing in spaced opposed relation, a refining plate assembly radially disposed on each said stationary head, a driving disc secured to said drive shaft centrally between said stationary heads, a pair of rotating heads mounted on said drive shaft. a refining plate assembly on each said rotating head in parallel juxtaposed relation to the refining plate assembly of the adjacent stationary head, said rotating heads being disposed one on each side of said driving disc, means connecting said driving disc and said rotating heads to provide rotation of said heads with said shaft while permitting axial movement thereof along said shaft, conveying means for directing stock to be refined into said casing between said juxtaposed refining plate assemblies, and means for adjusting the axial position of said rotating heads on said drive shaft, said latter means comprising cylinder-piston means on said shaft connected with each said rotating head, and a parallel hydraulic circuit operatively connecting said piston-cylinder means to equalize the positioning force on each said rotating head thereby permitting said rotating heads to float axially to balance the hydraulic cylinder-piston force against the axial force developed between the refining plate assemblies.

3. The invention as claimed in claim 2 wherein said rotating heads are each mounted on a sleeve disposed in concentric axially slidable relation with said drive shaft.

4. The invention as claimed in claim 3 wherein said cylinder-piston means connected with each said rotating head comprises a piston secured to said drive shaft, and a cylinder cooperatively disposed around said pis ton connected with the sleeve of said rotating head.

5. The invention as claimed in claim 3 including hydraulic fluid conduits in said drive shaft communicating with the cylinder chambers on either side of said piston, said hydraulic fluid conduits being arranged in parallel whereby a common fluid pressure will be simultaneously applied to the corresponding chamber of each said cylinder-piston means.

6. The invention as claimed in claim 3 including means for limiting the axial movement of each said rotating head.

7. The invention as claimed in claim 6 wherein said means for limiting the aixal movement of each said rotating head toward the adjacent stationary head comprises a stop ring secured to said shaft adapted for engagement by said rotating head sleeve.

8. The invention as claimed in claim 7 including a flanged stop nut connected to each said sleeve for engagement with the corresponding stop ring to limit axial movement of the rotary head away from the adjacent stationary head.

9. The invention as claimed in claim 2 wherein said conveying means for directing stock into said casing adjacent each said refining plate assembly comprises a cylindrical stock conduit concentric with said drive shaft and overlying each said sleeve, conveying elements mounted on each said sleeve, and a screw conveyor for introducing stock into said stock conduit.

10. The invention as claimed in claim 9 wherein said sleeve conveyor elements comprise a plurality of radially extending lugs, said lugs each having a pair of intersecting faces oppositely angularly inclined with respect to the drive shaft axis, the lug edge formed by said intersecting faces being directed toward the adjacent rotating head.

11. The invention as claimed in claim 9 including a steam discharge conduit for removing steam from each said stock conduit, said steam discharge conduit being axially outwardly spaced from said screw conveyor.

12. The invention as claimed in claim 2 wherein said means connecting said driving disc and said rotating heads comprises a plurality of axially extending lugs on 3,893,631 9 10 each side of said driving disc, and a plurality of lugs on casing comprises a plurality of easing sections, and

each said rotating head engaged with the lugs of said means for opening said casing sections to provide acdriving disc. cess to said refining plate assemblies.

13. The invention as claimed in claim 2 wherein said 

1. A disc type refiner comprising a base, a casing supported by said base, a drive shaft extending through said casing, means on said casing for rotatably supporting said drive sHaft, a pair of stationary heads mounted within said casing in spaced opposed relation, a refining plate assembly radially disposed on each said stationary head, a pair of rotating heads mounted on said drive shaft, a refining plate assembly on each said rotating head in parallel juxtaposed relation to the refining plate assembly of the adjacent stationary head, means connecting said drive shaft and said rotating heads to provide rotation of said heads with said shaft while permitting axial movement thereof along said shaft, conveying means for directing stock to be refined into said casing between said juxtaposed refining plate assemblies, and means for adjusting the axial position of said rotating heads on said drive shaft, said latter means comprising cylinder-piston means on said shaft connected with each said rotating head, and a parallel hydraulic circuit operatively connecting said pistoncylinder means to equalize the positioning force on each said rotating head thereby permitting said rotating heads to float axially to balance the hydraulic cylinder-piston force against the axial force developed between the refining plate assemblies.
 2. A disc type refiner comprising a base, a casing supported by said base, a drive shaft extending through said casing, means on said casing for rotatably supporting said drive shaft, a pair of stationary heads mounted within said casing in spaced opposed relation, a refining plate assembly radially disposed on each said stationary head, a driving disc secured to said drive shaft centrally between said stationary heads, a pair of rotating heads mounted on said drive shaft, a refining plate assembly on each said rotating head in parallel juxtaposed relation to the refining plate assembly of the adjacent stationary head, said rotating heads being disposed one on each side of said driving disc, means connecting said driving disc and said rotating heads to provide rotation of said heads with said shaft while permitting axial movement thereof along said shaft, conveying means for directing stock to be refined into said casing between said juxtaposed refining plate assemblies, and means for adjusting the axial position of said rotating heads on said drive shaft, said latter means comprising cylinder-piston means on said shaft connected with each said rotating head, and a parallel hydraulic circuit operatively connecting said piston-cylinder means to equalize the positioning force on each said rotating head thereby permitting said rotating heads to float axially to balance the hydraulic cylinder-piston force against the axial force developed between the refining plate assemblies.
 3. The invention as claimed in claim 2 wherein said rotating heads are each mounted on a sleeve disposed in concentric axially slidable relation with said drive shaft.
 4. The invention as claimed in claim 3 wherein said cylinder-piston means connected with each said rotating head comprises a piston secured to said drive shaft, and a cylinder cooperatively disposed around said piston connected with the sleeve of said rotating head.
 5. The invention as claimed in claim 3 including hydraulic fluid conduits in said drive shaft communicating with the cylinder chambers on either side of said piston, said hydraulic fluid conduits being arranged in parallel whereby a common fluid pressure will be simultaneously applied to the corresponding chamber of each said cylinder-piston means.
 6. The invention as claimed in claim 3 including means for limiting the axial movement of each said rotating head.
 7. The invention as claimed in claim 6 wherein said means for limiting the aixal movement of each said rotating head toward the adjacent stationary head comprises a stop ring secured to said shaft adapted for engagement by said rotating head sleeve.
 8. The invention as claimed in claim 7 including a flanged stop nut connected to each said sleeve for engagement with the corresponding stop ring to limit axial movement of the rotary head away from the adjacent statioNary head.
 9. The invention as claimed in claim 2 wherein said conveying means for directing stock into said casing adjacent each said refining plate assembly comprises a cylindrical stock conduit concentric with said drive shaft and overlying each said sleeve, conveying elements mounted on each said sleeve, and a screw conveyor for introducing stock into said stock conduit.
 10. The invention as claimed in claim 9 wherein said sleeve conveyor elements comprise a plurality of radially extending lugs, said lugs each having a pair of intersecting faces oppositely angularly inclined with respect to the drive shaft axis, the lug edge formed by said intersecting faces being directed toward the adjacent rotating head.
 11. The invention as claimed in claim 9 including a steam discharge conduit for removing steam from each said stock conduit, said steam discharge conduit being axially outwardly spaced from said screw conveyor.
 12. The invention as claimed in claim 2 wherein said means connecting said driving disc and said rotating heads comprises a plurality of axially extending lugs on each side of said driving disc, and a plurality of lugs on each said rotating head engaged with the lugs of said driving disc.
 13. The invention as claimed in claim 2 wherein said casing comprises a plurality of casing sections, and means for opening said casing sections to provide access to said refining plate assemblies. 